Two methods are commonly used to raise large objects, such as sunken ships, from the sea bed. Firstly, the object may be lifted directly with cables and a crane mounted on a suitable vessel. This is open to many objections. For very heavy objects, a large and very costly lifting vessel must be employed. The cables may tangle if more than one is needed to sustain the weight. At very great depths, the weight of the steel cables is a significant part of the total load. Steel cables have little compliance, and so will transmit wave movements directly to the load, considerably increasing peak stress. These last two problems can be overcome by using cables made of synthetic fibre with approximately the same density as water. However, these are costly, and, if they break, the large amount of stored energy can cause serious accidents.
The second lifting method employs air-bags. A balloon is attached to the load, and air is pumped into it, generating lift equivalent to the water displaced. A variation of this is the close all the apertures on a wreck, and fill it with air; the wreck itself then acts as its own balloon. Although it is simple and relatively cheap, this method suffers from being virtually uncontrollable. Normally, extra lift, over and above the weight in water, is required to break the object free from the bottom. Once the load starts to move upward, the air in the balloon expands, further increasing the lift. The rate of ascent therefore increases, until the load virtually leaps out at the surface. Since the air-bag usually has an open bottom, the air is often spilled at the surface, so the load descends to the bottom again.
There are other problems at very great depths. The air must be pumped down from the surface at a pressure at least equal to that at the sea bed; hence, powerful pumps and very heavy pressure hose must be used. Furthermore, the solubility of a gas is proportional to its partial pressure (Henry's Law), so a considerable proportion of the air actually supplied will be lost by dissolving in the sea-water.
The system proposed seeks to combine the simplicity and cheapness of the air-bag system with the excellent control of the direct lift method.